1. Field of the Invention
The present invention relates to polymeric materials usable in non linear optics. More precisely, it relates to a new type of material which, once suitably oriented, is capable of generating with high efficiency the second harmonic of an electromagnetic wave whose wave length may be between 300 nm and 2000 nm.
2. Description of the Prior Art
At the present time, the most widely used materials for obtaining the second harmonic generation phenomenon, that is to say for obtaining a wave of pulsation 2.omega. from a wave of pulsation .omega., are monocrystal minerals such as lithium niobate or lithium dihydrogenophosphate.
It is only about ten years or so ago that it was realised that organic materials could lead to very intense non linear phenomena, very often greater than those obtained with mineral products.
Organic materials in second harmonic generation have the characteristic of a very great dissymmetry of electronic density on the molecular scale. This is the case for example of 4N dimethylamino-4'-nitrostilbene (DANS). But so that the compound gives rise to the second harmonic generation phenomenon, the molecular arrangement must in addition keep on the macroscopic scale the electronic density dissymmetry which exists on the molecular scale. To satisfy this condition, it is necessary to obtain non centrosymmetric monocrystals. However, the rate of success in obtaining such crystals is only 20% with non chiral compounds. In fact, the molecules such as the DANS have a high permanent dipolar moment and pair up in a head to tail arrangement, which leads to a cestrosymmetric system.
The center of symmetry may be eliminated in a monocrystal formed from molecules having a high hyperpolarizability but pairing up head to tail by dissolving it in a nematic polymer matrix with side chains. This has been described by G. R. Meredith, J. G. Van Dusen and D. J. Williams in the review Macromolecules, 1982, vol. 15, pp 1385-1389. The mixture thus formed is heated above the vitreous transition temperature (Tg) of the polymer. The non centrosymmetric orientation is obtained by means of a DC electric field. It is then "frozen" in the vitreous state by cooling the whole under an electric field. In such a procedure, the molecules which are active in generating the second harmonic are placed parallel to the side chains of the polymer, these latter being oriented parallel to the electric field.
In the article mentioned, it is a copolymer which was used as orientating matrix (or host), the DANS being used as a compound capable of generating the second harmonic. The material obtained has a susceptibility of the second order .chi.(2) of 6.times.10.sup.-9 esu (electrostatic unity) using an electric field of 0.6 V/.mu..
The major drawback of this material comes from its method of manufacture. The low solubility of the molecules capable of generating the second harmonic in the mesomorphic polymers leads to low intensities of the wave at pulsation 2.omega..
To overcome these drawbacks, the invention proposes increasing the efficiency of the material by fixing, by means of a covalent connection, the second harmonic generating entity and the mesogenus group on the same polymeric skeleton. This allows materials to be obtained with a high content of second harmonic generating functions.